1. Field of the Invention
The present invention relates to an electronic device having a touch panel and related method of processing information on the electronic device, and more specifically, to an electronic device having a touch panel and related method capable of determining a touch position by processing of analog signal using a demodulation signal without phase compensation.
2. Description of the Related Art
In order to facilitate the carrying and utilization, touch-panels which users can touch directly have become a new trend in market development. For example, a liquid crystal display (LCD) which is applied to a personal digital assistant (PDA) is usually combined with a touch-panel to omit a keyboard or functional buttons. The touch-panel generates electric signals in response to a touch thereon to control image display of the LCD and implement function control.
Please refer to FIG. 1 showing a conventional electronic device 10 with a touch panel 11. The touch-panel 11 detects a touch location and strength of a finger or pen on a sensing array 12 of the touch panel 11. When a user touches the touch panel 11 with his finger or with a pen, the value of resistance or the value of capacitance of a sensing element of the sensing array 12 will vary. For instance, when a user presses down a resistive touch-panel 11 made of flexibility material with his finger, the distance between upper and lower electrodes becomes shortened, and the value of resistance between the two electrodes is altered. Or, when a user presses down a capacitive touch-panel 11 with his finger, the capacitance between two electrodes is affected due to the transmission characteristics of the human body, and the value of capacitance between the two electrodes is altered. Therefore, the touch location and strength of a finger or pen can be detected by detecting the variation in resistance or capacitance value.
The sensing array 12 comprises strip conductive conductors in two groups, one of which is in X direction and the other of which is in Y direction. The two groups are interlaced with each other. Or, the sensing array 12 comprises concentric and radial conductors arranged on polar coordinate. Each intersection of the two conductors substantially forms a resistance or capacitance element. When a conductor of a certain row is switched on by a driving signal via a multiplexer 16 from a control unit 14, it is allowed to detect the touch intensity of the row by serially or simultaneously measuring sensing signal values of each intersection of a column with the row by the control unit 14 through a multiplexer 18. By serially or simultaneously measuring sensing signal values of each row, an intensity matrix is obtained to determine a finger's touch location and strength.
However, sensing elements are easy to be disturbed by noise, which increases difficulty determining if variations in capacitance values result from a finger's touch or attribute to environmental noise. Take a 50 pF capacitor for example. The variation caused by a finger is 1 pF. When the 50 pF capacitor is charged to a voltage level of 2V, the voltage variation caused by the finger is approximately 40 mV, and the disturbance of noise is roughly tens of mV as well. As a result, the signal-to-noise ratio (SNR) is not strong enough, which leads to being inclined to misjudgment and even leads to misjudgment of ghost effects while there are no touches.
In addition, current touch-panels probably go with many functions in wireless communications (infrared transmission or bluetooth transmission), match with backlight liquid crystal (LC) panels, and so on. So sources of noises received by the touch-panel 11 are quite complicated, such as flicker noise (1/f noise), white noise, power noise, 50/60 Hz noise, communication microwave generated by infrared or bluetooth transmission, backlight noise, etc. In general, a low pass filter is utilized to filter high-frequency noise. For low-frequency noises such as flicker noise and 50/60 Hz noise, although low-frequency elements can be filtered out when a low pass filter is designed at lower cut off frequency, response time will be elongated as well. For instance, if the cut off frequency of a low pass filter is operated at 10 Hz to filter out 60 Hz noise, response time will delay about 0.1 second. The side effect thereof corresponds to a 0.1 second delay of conductor drawing speed. That is, a delay in response time for an application program (e.g., picture dragging) may occur after a finger touch, resulting in inconvenience in application.
In order to solve the above-mentioned problem, a modulation signal at frequency f1 is fed to the sensing element. Then a demodulation signal is used for demodulating a sensing signal produced by the sensing element, so as to generate signals at frequency (f1+f2) and frequency (f1−f2). By means of a low pass filter with a cut off frequency of (f1+f2)/2, high frequency component over (f1+f2)/2 is filtered out while the low frequency component below (f1+f2)/2 is obtained. When f1=f2 is chosen, the low-frequency component is a direct current (DC) term, i.e., a required signal. Measuring variations of the DC term is just equivalent to measuring variations of a capacitance as a finger touches. Because the modulation and demodulation technique is able to be operated on the high frequency (HF) wave bands, low-frequency noise disturbance can be thus avoided. But the conventional modulation and demodulation technique needs highly complicated analog circuit layout. Besides, additional isolation circuits are required when analog circuits and digital circuits coexist. In this way, costs will increase.
Please refer to FIG. 1 and FIG. 3. FIG. 3 depicts a functional diagram of the control unit 14 and touch panel 10 in FIG. 1. As shown in FIG. 1, trace A and trace B indicate different traces when measuring point A and point B. Amplitudes of sensing signals are varied according to different trace delays. The greater trace delay causes the smaller amplitude of the sensing signal, thereby, reducing the dynamic range of measuring variation. In a large-sized sensing array, a problem like trace delays becomes more serious. Therefore, conventionally, a use of a phase calibrator 22 is to measure all trace delays and makes phase compensation for each point. For example, when the sensing array 12 is operated under an un-touched state, a driving signal which is a square waveform from a signal generator 24 is sequentially fed to the sensing array 12 conductor by conductor, Amplitude of sensing signals are varied dependent on the trace delays. During non-touch period, phase calibrator 22 can produce square waveforms of the same frequency in diverse phases, and demodulate the received sensing signals with the square waveforms in diverse phases to obtain an autocorrelation relationship between each phase and the sensing signal. As it is, a phase corresponding to the maximum value is the required phase compensation value for the point. Afterwards, phase compensation values of all of the points are sequentially produced to generate a look-up table. In the following measurements, it is allowed to look up the table directly to act as phase compensation or to just choose several points to measure, such as four points at the four corners of the periphery, to produce phase compensation values in the manner of linear interpolation. However, phase calibration and compensation are complicated and computational consuming.